The present invention relates to microwave cavity tuning for gas absorption spectrometry, and more particularly it relates to variable tuning of such a cavity by means of a piezoelectric or a magnetostrictive transducer.
In microwave absorption spectrometry, it is necessary to vary the tuning of a microwave cavity in order to detect an absoprtion peak. Instrumentation known in the art to accomplish this include Stark spectrometer systems with electronic frequency scanning, and a system including a Fabry-Perot confocal resonator using a stepping motor to adjust the resonator spacing to carry out frequency scanning. However, for practical microwave absorption spectrometry of pollutants it is desirable to examine numerous successive samples, with each sample being examined for various pollutants over a range of frequencies, with sensitivity and without changing the chemical or physical properties of the sample being examined. In addition, the instrument used for such spectroscopy should be easily portable from site to site. Although a Stark spectrometer is rapid, it tends to have a low Q and therefore a low sensitivity, it requires a very high operating voltage which may lead to voltage breakdown and disruption of the analysis and destruction of the equipment, it tends to change radicals in a fast radical study due to the required high voltage and it is a large, expensive laboratory instrument with a very sophisticated electronic system and is not easily portable. Another known gas absorption spectrometer system utilizes a Fabry-Perot microwave cavity wherein frequency scanning is accomplished by varying the distance between opposing walls of the microwave cavity by means solely of a servomotor. However, due to the mechanical inertia of the servomotor and the tendency of a servomotor to hunt, such an arrangement is slow; and when many samples are to be sensitively analyzed for a large number of pollutants, such a servomotor system becomes so cumbersome and time consuming as to be impractical.